Gaps between liquid and obstacles in liquid simulation
The obvious answer is to increase the resolution, but while this will make the problem less noticeable, there will still be some of this "mushrooming effect". Furthermore, with increased resolution the behaviour of the fluid changes. It becomes more "splashy" if you know what I mean. I baked your sim at a resolution of 300, and the fluid splashed all over the place.

What you will then have to do is that once you have baked the sim, you move your domain downwards (or the obstacle upwards) and maybe do some slight stretching of the domain until these artifacts disappear.

Anything else won't matter. However, about some of your comments:

You say that it looked good with a resolution of 50, but I baked it and it looked horrible. Most likely your observation was due to some other factor, such as not having applied the subsurf modifier.

Yes, about the subsurf modifier. The fluid will ignore the modifier unless it is applied. It will just react to the obstacle mesh as it is. In your case, this might have covered up the problem, as the subsurf decreases the volume of the "canal". A lot of fluid will thus be hidden inside the obstacle, so to speak.

Good luck!